The muscle pdf - د. حنان

The muscle

Objectives

1-Recognize the meaning of summation of contraction

and its types.

2-detrmine the effect of changing length on skeletal

muscle tension.

3-Differntiate between the 2 types of muscle fibers.
4- Describe the motor unit and how its type affects the

function of the muscle?

5- Discuss the energy sources for the muscle and for

what it is used?

6-Outline the abnormalities in muscle fiber contractions

The summation of contraction

It means the adding together of individual twitch

contractions to increase the intensity of overall
contraction.

Because the contractile mechanism has no

refractory period.

Repeated stimulation, before relaxation can

produce additional activation of the contractile
elements .

The response will be added to that already present,

this is called

"summation of contraction".

It depends on the frequency of stimulation. It occurs in
2 ways:

1 – Multiple fiber summation (increasing the

number of motor units contracting at the
same time).

2- Frequency summation and tetanization, with

rapid  repeated  stimulation,  before  any
relaxation  occurs  and  the  response  fuses  into
one  continuous  contraction  and  the  whole
contraction  appears  to  be  smooth  called
Tetanus  (

by increasing the frequency of

contraction).

During tetanus the tension developed is 4 times than
the individual contraction.

At slightly higher frequencies, the strength of the

muscle contraction reaches maximum, so any

additional increase in frequency beyond that

point has no further effect in increasing the force

of contraction,

This is because enough Ca ions are maintained in

the sarcoplasm ,even between action potentials

so will not allow relaxation to happen.

But if a lower frequency is used, there will be a

period of incomplete relaxation between the

summated stimuli; this condition is called

incomplete tetanization or clonus.

3- The CNS sends weak signals, so smaller motor

units contract than the larger ones, the
strength will increase, larger and larger motor
units will be excited (large motor units have
more contractile force than the smaller ones).

Effect of muscle length on the force of

contraction:

The tension develops in a muscle when it is

stimulated to contract

isometrically

; this is

called the total tension.

When the length of the unstimulated muscle

fiber is changed this is called passive tension..

Passive tension curve

is the curve plotted to

include the changes in tension against the

changes in the muscle length. It shows that, as

the length increases, it rises sharply.

The total tension curve

shows the total tension

against the passive tension, we will see a sharp

increase up to a maximum value, then the curve

declines.

If we measure the distance between the two curves

we will see

the active tension curve

which is

similar in shape with the total tension curve but

has a lower peak.

From these curves we will see that the maximum tension

occurs when the muscle length is at its resting length i.e.

the sarcomere length is about 2 micrometer (relaxed state).

Shortening or contraction of the muscle will reduce this

tension. This is because the maximum number of

interaction between actin and myosin occurs at the resting

length .So shortening of the muscle fiber, causes smaller

sarcomere and actin filament overlap, so less actin to

myosin interaction sites

Stretching of muscle fiber (increase in its length) will cause

less actin to myosin interaction i.e. bigger sarcomere with a

large gap in the middle ..

Energy sources and metabolism:

Contraction of the muscle depends on energy supplied by ATP.

In general the source of energy is the metabolism of
carbohydrates and lipids.

Most of the energy is required for physical activity

(contraction, relaxation) or walk along mechanism, and

small amounts are required for:

1- Pumping of Ca ions from the sarcoplasm to the

sarcoplasmic reticulum after the contraction is over.

2- Regeneration of ATP.
ATP split to ADP, then ADP rephosphorlated to ATP.
3- Removal of lactic acid.
4- Heat production.

Sources of energy for the rephosphorlation

1- Substance called phosphocreatine (high

energy phosphate bond).

ADP+P→ATP
This compound supplies energy to the muscle

at rest .During exercise this compound
hydrolyse at the junction between actin and
myosin releasing energy ,this reaction is
catalyzed by the enzyme phosphorylcreatinine
at the mitochondria and myosin heads.

Glucose

: it is supplied by the blood and undergoes

series of reactions forming finally Co2, H2o and Energy.

Glycolysis of glycogen

stored in the muscle cells:

enzymatic break down of glycogen to pyruvic which

has 2 pathways in the presence of O2, it enters citric

acid cycle (crips cycle), then the respiratory chain to

form co2, H2O and large amount of energy, (aerobic

Glycolysis). But in the absence of O2(like in prolong

contraction), pyruvate is reduced to lactate and lactic

acid and small amount of energy.

The free fatty acids(FFA

) (gives double the energy that

glucose gives)

skeletal muscles take the FFA in the blood and oxidized

to give Co2, H2o and ATP (the use of FFA mainly at rest

and during recovery after contraction

The oxygen debt mechanism:

During exercise

the blood vessels dilates to provide

enough O 2 for the muscle, the energy is supplied by

aerobic glycolysis.

If the exercise is sever

or continues for longer

periods,

the anaerobic glycolysis

contribute

also to provide energy.
It is self limiting ,because lactic acid will diffuse

to the blood it will lower the PH ,accumulate
in the muscle causing muscle exhaustion ,

So after the exercise there will be a period of

hyperventilation to produce the extra amount
of O2 in order to remove the lactic acid and to
rebuild the storage of ATP and
phosphorylcreatinin . This extra amount of O2
taken to replace the demand required more
than that supplied by the aerobic glycolysis
during exercise is called the

oxygen debt

.mechanism

Trained persons need less period of

hyperventilation because they have

smaller

oxygen debt mechanism

(i.e. he has

endurance, he can use the muscle to perform
the job better).

Heat production in the muscle:

This is produced as:
1-

Resting heat

: liberated during resting stage.

Initial heat

which is produced during activity and it is

divided into:

Activation heat

: produced during contraction, from the

actin –myosin interaction.

b -

Shortening heat

: produced during shortening

(isotonic contraction), due to the changes in muscle

fiber structure during shortening.

In isotonic contraction there are both activation and

shortening heat but in isometric there is only activation

heat.

3- Recovery heat

: liberated by the metabolic

processes that restore the muscle fiber to its
precontraction state and it is rather equal to
the initial heat.

4- Relaxation heat

: liberated because work

should be done to return the muscle to its
original length (after isotonic contraction) in
addition to the recovery heat.

Types of muscle fibers

3 types according to the differences in enzyme activity,

metabolism and contractile properties:

1- Type I fibers

: these are darker than other muscles

called

Red muscles

or the slow fibers, they response

slowly and have longer duration of action(resist
fatigue). They are specialized for long slow sustained
contraction, supplied by slow conducting fibers. e.g.
muscles in the back and in the lower limbs which are
used to maintain posture. They are small fibers, have
more extensive blood supply to supply high oxygen,
and high number of mitochondria and large number of
myoglobulin.

Type II b:

called the white muscles or the fast fibers, they

are innervated by large mylinated fibers, they have short

duration of action(fatigue quickly) .So they are specialized

for fine skilled movement e.g. muscles of the hand and the

extra-ocular muscles. They are large fibers and have

extensive sarcoplasmic reticulum for rapid release of Ca

ions, large amounts of glycolytic enzymes for rapid release

of energy, they have less extensive blood supply and fewer

mitochondria and less myoglobulin.

Type II a

: this is rare type in human, has properties similar

to type I, and other properties to type II.

The motor units

The motor unit means all the muscle fibers

innervated by a single nerve fiber .i.e. the axon of
a single motor neuron divides to supply many
muscle fibers. There are 2 types of motor units:

1- Small motor units

: contain 3-6 muscle fibers,

concerned with fine graded, precise movement,
like movements of the hand. They are Small
muscles that react rapidly and whose control
must be exact have more nerve fibers for fewer
muscle fibers (2-3 muscle fibers for each motor
unit, e.g. laryngeal muscles).

large motor units

:contain usually 120-165

muscle fibers ,like muscles of the back ,for the
sustained form of activity. These are large
muscles that do not need fine control (e.g.
soleus muscle), may have several hundreds of
muscle fibers in the motor unit.

Each motor unit is of one type i.e. innervates

one type of muscle, but when a nerve to slow
muscle is cut and replaced by a nerve to fast
muscle ,the slow muscle after a period of time
becomes fast.

The muscle fibers in each motor unit overlap

other motor units in microbundles of 3-15
fibers. This interdigitation allows the separate
motor units o contract in support of one
another rather than as individual segment.

Denervation:

Means the deprivation of muscles from the nerve supply, the following

effects will happen:

1- Immediate loss of muscular activity called flaccid paralysis.
2- Abnormal excitability of muscle fiber with increase sensitivity to

circulate Ach (deneravation hypersensitivity), this result in fine

irregular contraction of individual muscle fiber (fibrillation).

Up to these 2 points, if the nerve fiber regenerates these 2 effects

will disappear.

3- Atrophy of the muscle: prolong denervation results in degeneration

of the muscle fibers and replacement by fibrous tissue, this result in

reduction of muscle size called wasting (because of decay of

contractile proteins). If after 2 months the nerve supply back, full

return of function ,but after 3 months the return back of muscle

function is less, and after 1-2 years, no return of muscle function.

All these features are of lower motor neuron lesions which is the

effect from the spinal cord to the muscle. Above that is called upper

motor neuron lesion.

Contracture:

This is persistent sustained contraction of the muscle without an action potential,

causing no relaxation. It occurs due to the increase level of extracellular calcium.

Rigor:

When the muscle fiber is completely depleted of ATP and phosphoryl creatinin, they

develop a state of complete contraction causing increase rigidity after death

because no ATP available for relaxation ,in this state Rigor mortis ,almost all the

myosin heads attached to the actin in an abnormal fixed and resistant way.(it

occurs more rapidly in high temperature).

Muscle fatigue:

Prolonged and strong muscle contraction lead to muscle fatigue. This results from:
-Depletion of glycogen stores in the muscle (inability of the contractile and metabolic

processes to continue supplying the same work).

-Diminished transmission in the NMJ .
- Interruption of blood supply or flow results in nutrient and O2 deficiency.